William J. Gambogi

A Comparison of Key PV Backsheet and Module Performance from Fielded Module Exposures and Accelerated Tests

The performance of photovoltaic (PV) modules and their component materials under the stresses in the outdoor environment is the most important indicator of reliability, durability, and safety of PV modules throughout their service life. Tedlar polyvinyl fluoride films have been a key component in backsheet component structures and used in PV modules that have been in the service environment for over 25 years. The performance of fielded modules and components can be compared with the performance in durability tests using other backsheet materials including polyethylene terephthalate. Accelerated testing protocols are described including UV exposure based on solar irradiance in different climates and relevant albedo levels for exposure of the back of PV modules. The change in critical performance properties in durability tests including damp heat and UV is compared with backsheets that are extracted from fielded modules. Analysis of mechanical and chemical properties of the inner and outer layers of backsheets that are removed from fielded modules is examined. Area-specific coring techniques that are followed by layer composition analysis were also used to understand fielded module failure mechanisms/defects. The impact of extended damp heat and UV on backsheet properties and module performance that is observed in the field is further quantified through measurement of mechanical, optical, electrical, and permeability properties of the backsheet and power, electrical insulation, and physical properties of the modules. Correlations between field performance and accelerated testing are demonstrated. The first results for sequential and combined stress testing are described and compared with the field performance. A comparison of two large and diverse sets of modules from two different locations (EU and Japan) is discussed, and the power loss with field exposure period and the impact of backsheet is described.

15.3: Invited Paper: Volume Holographic Components for Display Applications

Volume holographic components are being used to enhance the performance of a variety of liquid crystal and projection display systems. Holographic components address the ever-present need within display systems for more efficient light management. With proper design, holographic components address this need with higher light throughput, reduced thickness and weight, reduced energy consumption, and additional opportunities for unique system functions. n n n nThis paper describes design parameters and examples of improved devices by introducing the unique features of diffractive holographic components. These features include angular and spectral selectivity, high efficiency, and multiple optical functions designed into single components. Holographic reflectors, front diffusers, and compensations films have been developed and used in direct view liquid crystal displays. Holographic color filters and projection screens have also been developed based on this technology. For these applications the holographic component and system will be discussed. Finally, design considerations will be discussed that relate to the development of new display systems and components.

Assessment of PV module durability using accelerated and outdoor performance analysis and comparisons

Photovoltaic modules have been operating in the outdoor environment for more than 30 years now. These modules have been exposed to a wide range of stresses including UV and visible radiation, high and low temperatures, seasonal and diurnal temperature variations, internal electric field, localized heat, moisture including rain, humidity and condensation, abrasion and other mechanical stresses. Over these 30 years, qualification requirements have been imposed to identify infant mortality failures in module design and materials, but much work remains in understanding the durability issues related to module performance and safety. In this paper, we will review analysis of modules taken from the field and discuss changes in the module performance as they relate to materials performance and changes. We will also discuss our work in simulating these stresses in accelerated durability testing and compare changes in module performance and safety to those observed in the field. Finally, we will review progress in simulating outdoor conditions using combinations of stresses applied to the module to better simulate outdoor conditions.

Analysis of glass-glass modules

Glass-Glass modules are gaining popularity for bifacial application and have believed advantages over PV modules with polymeric backsheets. Frameless glass-glass modules are promoted as PID-free, resistant to solvents, fire, and load stress, and capable of higher system voltages. We have found glass-glass modules run at higher operating temperature than Glass-Flex modules, and this reduces power output. Field power output results will be presented. Impermeable glass traps chemical byproducts, and faster power degradation from corrosion has been documented. Delamination has been observed in the field with glass-glass modules. A new accelerated test replicates this delamination. PID testing results will be presented comparing Glass-Glass and Glass-Flex modules.

Comparison of higher irradiance and black panel temperature UV backsheet exposures to field performance

The need for faster PV qualification tests that more accurately match field observations is leading to tests with higher acceleration levels, and validating the new tests through comparison to field data is an important step. We have tested and compared a wide panel of backsheets according to a proposed new backsheet UV exposure qualification standard from the International Electrotechnical Commission (IEC). Weathering Technical Standard IEC 62788-7-2 specifies higher irradiance and higher black panel temperature UV Xenon exposures. We tested PVF, PVDF, PET, PA and FEVEbased backsheets in glass laminates and simple backsheet coupons in UV exposure condition A3 (0.8W/sqmnm@340nm and 90° C BPT) We find mild yellowing with no mechanical loss in the original lower intensity ASTM G155 0.55 W/sqm-nm 70C BPT exposure condition. The new A3 exposures creates mechanical loss in sensitive backsheets, with no effect on known durable backsheets. Results from the new exposure are closer to field mechanical loss data.

Compensating a Twisted Nematic Liquid Crystal Display with Matched Wavelength Dispersion Retarders

We show that holographic retarders using form birefringence can be designed to allow accurate matching of the dispersion of a liquid crystal device. A twisted nematic (TN) liquid crystal display (LCD) is compensated using stacked, dispersion matched, holographic retarders and the results are compared with a TN-LCD compensated by a single, splayed optic axis retarder on both sides of the LC layer. The new compensator design is shown to have excellent qualities, and the potential to surpass previous designs.